Field of The Invention
The invention relates to the domain of parts formed by stamping or extrusion for automobiles, particularly parts assembled by remote laser welding without filler wire. More particularly it relates to aluminium alloy parts in the AA6xxx family according to the “Aluminum Association” designation, to which hardening elements have been added and for use in manufacturing by stamping of inner parts, structural parts or stiffeners for the body-in-white of automobile vehicles.
Description of Related Art
In preamble, in the following and unless mentioned otherwise, all aluminium alloys concerned are designated by the designations defined by the “Aluminum Association” in the “Registration Record Series” that it publishes regularly. Unless mentioned otherwise, indications about the chemical composition of alloys are expressed as a percent of weight based on the total weight of the alloy; “ppm” means parts per million by weight.
The definitions of the metallurgical tempers are indicated in European standard EN 515.
Static mechanical properties in tension, in other words the ultimate strength Rm, the conventional yield stress at 0.2% elongation Rp0.2, and the elongation at failure A %, are determined by a tensile test according to standard NF EN ISO 6892-1.
Aluminium alloys are increasingly used in the manufacture of automobile vehicles because their use can reduce the weight of vehicles and thus reduce fuel consumption and the release of greenhouse gases.
Aluminium alloy sheets are currently used for the manufacture of many “body-in-white” parts including skin and bodywork parts (or external bodywork panels) such as the front wings, roof, bonnet (hood), boot (trunk) and door skins, inner parts for example such as inner door, wing, rear door and bonnet parts; and finally structural parts for example side members, front shields, load bearing floors and front, middle and rear foot rests.
Many skin and inner parts are already made using aluminium alloy sheets.
For this type of application, there are several sometimes antagonistic required properties, such as:
high formability in the delivery temper T4, particularly for stamping operations,
a controlled yield stress in the delivery temper of the sheet to control elastic springback when shaping,
high mechanical strength after cataphoresis and baking of paints to obtain good mechanical strength in service while minimising the weight of the part,
good energy absorption capacity in case of impact, good behaviour in the various assembly processes used in automobile bodywork such as spot welding, laser welding, bonding, and even clinching or riveting,
good resistance to corrosion, particularly intergranular corrosion, stress corrosion and filiform corrosion of the finished part,
compatibility with recycling requirements for fabrication waste or waste from recycled vehicles,
an acceptable cost for production in large series.
There are no solutions in prior art consisting of monolithic sheets that can be assembled by remote laser welding without added filler wire and that have mechanical, formability and corrosion characteristics similar to aluminium alloy sheets currently used in the automobile industry.
Furthermore, known solutions particularly to reduce the sensitivity of aluminium alloys to cracking and that could more generally improve their weldability are to increase the contents of silicon to above 2%, magnesium to above 5%, and copper to above 6% (see FIG. 1).
In the case of alloys in the AA6XXX series, a filler wire is used to provide good resistance to cracking during laser welding; the filler wire is composed of an alloy in the AA4XXX series with high silicon content (for example 12%) or an alloy in the AA5XXX series. It is also known that elements such as Titanium and Zirconium can be used to refine the solidification structure and consequently reduce the sensitivity to cracking during laser welding, as reported in “Current issues and problems in laser welding of automotive aluminum alloys”, H. Zhao, D. R. White, and T. DebRoy, International Materials Reviews, Volume 44, Issue 6 (1 Jun. 1999), pp. 238-266, from which FIG. 1 is extracted.
Although, as mentioned above, there is no known solution in prior art for assembly of a monolithic sheet, in other words a non-composite sheet composed of two co-rolled alloys or obtained by a “bi-alloy” casting, for assembly of automobiles by laser welding, a monolithic sheet has been developed by “Sky” for application to arc welding using the TIG and MIG processes and is disclosed in application U.S. Pat. No. 4,897,124. The range of composition of said sheet is defined on FIG. 2, with an Fe content of between 0.05% and 0.5% and at least one element from the following group: Mn with a content of less than 0.6%, Cr with a content of less than 0.3% and Zr also with a content of less than 0.3%. Improved weldability is claimed, together with improved formability and resistance to corrosion.
Furthermore, a multilayer product has been developed by “Novelis” as reported in “Advanced Aluminum 5XXX and 6XXX for complex Door Inner Panels and Consideration for an Aluminum-specific Design”, A. Walker, G. Florey—Novelis Switzerland SA; Bad Nauheim—Doors and Closures in Car Body Engineering 2014 and “Laser Remote Welding of Aluminum without filler”, R. Brockmann (Trumpf), C. Bassi (Novelis) 2012 Apr. 19.
This is an assembly composed of a core sheet made of “Novelis 6200” alloy, cladded with sheets made of an alloy in the AA4XXX family (with a 12% content of Si, slightly below the Al—Si eutectic [see Laser Remote Welding of Aluminum without filler; R. Brockmann (Trumpf), C. Bassi (Novelis) 2012 Apr. 19]) on the surface. Its trade name is 6200RW or “Novelis Advanz s200 RW”. It has improved weldability during remote laser welding without filler wire, with no crack in the welded joint as stated in “Laser Remote Welding of Aluminum without filler”, R. Brockmann (Trumpf), C. Bassi (Novelis) 2012 Apr. 19. However this type of monolithic product is not ideal in terms of cost and recycling.
Patent application JP2006104580 discloses an aluminium alloy sheet in the 3XXX series with good pulsed laser weldability, with composition as a % by weight, Si: >0.20 to 0.60, Fe: 0.25 to 0.55, Cu: 0.10 to 0.35, Mn: 0.9 to 1.5, Mg: 0.25 to 0.55, remainder aluminium and inevitable impurities, the total value of Si, Fe, Cu and Mg being less than or equal to 1.5% by weight. These sheets made of 3XXX alloy do not have the required mechanical characteristics.
Patent application US2005/0155676 describes safety or structural parts obtained by die-casting made from an alloy containing Si: 2 to 6, Mg<0.40, Cu<0.30, Zn<0.30, Fe<0.50, Ti<0.30, at least one element to reduce the bond with the mould such as Mn (0.3-2), Cr (0.1-0.3) Co (0.1-0.3) V (0.1 to 0.3) or Mo (0.1-0.4) and at least one element to modify the eutectics such as Sr (50-500 ppm), Na (20-100 ppm) or Ca (30-120 ppm). These cast products do not have the required geometric and mechanical characteristics
Patent application JP 1995109537 discloses a hypo-eutectic Al—Si alloy that can be used for cast, extruded and forged products that have the following composition as a % by weight, Si: 3.3-5.5, Mg: 0.2-0.7, Ti: 0.01-0.2, B: 0.0001-0.01, Fe≤0.2, P≤0.005 and Ca≤0.005 and satisfying the relation P/Ca≤1.0 by weight. These products do not have the required geometric and mechanical characteristics
Patent application US2005/0100473 describes aluminium alloys and cast products containing the following % by weight, Si: 4-12, Cu<0.2, Mg: 0.1-0.5, Ni: 0.2-3.0, Fe: 0.1-0.7, Ti: 0.15-0.3 and the remainder aluminium and impurities. These cast products do not have the required geometric and mechanical characteristics